Implantable medical devices are available for treating cardiac arrhythmias by delivering anti-tachycardia pacing therapies and electrical shock therapies for cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, conventionally senses a patient's heart rate and classifies the rate according to a number of heart rate zones in order to detect episodes of tachycardia or fibrillation. Typically a number of predefined rate zones are defined according to programmable detection interval ranges for detecting slow ventricular tachycardia, fast ventricular tachycardia and ventricular fibrillation. Intervals between sensed R-waves, corresponding to the depolarization of the ventricles, are measured. Sensed R-R intervals falling into defined detection interval ranges are counted to provide a count of VT or VF intervals. A programmable number of intervals to detect (NID) defines the number of tachycardia intervals occurring consecutively or out of a given number of preceding event intervals that are required to detect VT or VF.
Tachyarrhythmia detection may begin with detecting a fast ventricular rate, referred to as a rate- or interval-based detection. Once VT or VF is detected based on rate, the morphology of the sensed depolarization signals may be used in discriminating heart rhythms to improve the sensitivity and specificity of tachyarrhythmia detection methods. Before a therapy decision is made, tachyarrhythmia detection may further require discrimination between SVT and VT using cardiac signal waveform morphology analysis, particularly when a fast 1:1 atrial to ventricular rate is being sensed. However, the sensitivity of tachyarrhythmia detection algorithms relying on rate-based detection as a primary detection method are limited to the reliability of sense amplifiers in accurately sensing P-waves and R-waves and by the selection of the rate zone thresholds for VT and VF detection. Rate-based arrhythmia detection schemes that rely primarily on P-wave and R-wave sensing are subject to limitations due to oversensing and undersensing of depolarization signals, which can result in either overestimating or underestimating the actual heart rate. Inadequately programmed rate zone thresholds can also cause over- or under-detection of tachyarrhythmias that might be responsive to ICD therapy. Undersensing of VF, e.g. due to undersensing of ventricular signals or other factors, can occur. The highest priority goal of an ICD is generally achieving high sensitivity to VF detection, since sustained VF is a life-threatening condition and requires prompt detection and treatment.